The present disclosure relates to implantable medical devices, and more particularly implantable medical devices that stimulate the neural system, e.g. cochlear stimulators (referred to hereafter as a “neural stimulator” or simply as a “stimulator”). Even more particularly, the disclosure relates to a neural stimulator electrical model that measures evoked compound action potential (eCAP) artifact, and further to systems and methods for eliminating a slow decaying artifact in the eCAP recordings in order to increase the accuracy of the eCAP measurements.
In an implantable medical device, particularly an implantable neural stimulator, there is a need to measure internal voltages, determine electrode impedances, determine output stimulus linearity, sense and measure biological responses to an electrical impulse, as well as to monitor and measure other biological activities that are associated with or occur coincident with the operation of the device.
For example, in order to measure a biological response to an applied stimulus (i.e., an “evoked response”), there is a need to deal with the presence of the stimulus artifacts which accompany any applied stimulus. Having the capability of sensing and monitoring the evoked response to an applied stimulus provides a very valuable tool for setting the stimulus parameters at an appropriate level for a given patient. However, sensing the evoked response has been difficult since it is such a small signal compared to the stimulus artifact.
Profound deafness due to sensorineuronal hearing loss can be alleviated with varying degrees of success, using a neural stimulator known as a cochlear implant (CI) prosthetic. This device includes both external (non-implanted) and implanted portions. The implanted portion comprises an implantable cochlear stimulator integrally attached to a cochlear electrode lead. The implanted cochlear stimulator is affixed to the temporal bone during surgery, and the electrode lead is positioned in the cochlea along the basilar membrane. The electrode lead includes, at its distal end, a multiplicity, e.g., sixteen, spaced-apart electrodes that may be inserted into a human cochlea, any one of which may be activated for application of an electrical stimulus to cochlear tissue. The electrodes are stimulated in a tonatopic manner to elicit a response in the auditory nerve.
The modern cochlear implants contain amplification and digitization circuitry which is sufficiently accurate to allow monitoring of the auditory nerve response evoked by a stimulus pulse. These evoked compound action potentials (eCAPs) are used in research settings to study the pathology of hearing loss and benefits of cochlear implants. These measurements are also commonly recorded in the clinical setting in order to determine the proper parameters for fitting the CI to benefit a particular patient. Evoked compound action potentials measurements are also known as neural response imaging (NRI) and neural response telemetry (NRT). (NRI-measurements are described, e.g., in U.S. Pat. Nos. 6,157,861 and 6,195,585, incorporated herein by reference, and relate, in general, to monitoring a response evoked by application of a stimulus pulse.)
An evoked compound action potential occurs at approximately 200 μs after the beginning of the stimulation pulse. The neural signals recorded in this manner have typical amplitudes of 100 to 1000 μV. Evoked compound action potential measurements are often seen superimposed on a decaying artifact. As Klop et al. point out, this artifact has been observed to follow a double exponential [Klop, W. M. C., et al., “A new method for dealing with the stimulus artefact in electrically evoked compound action potential measurements”. Acta Oto-Laryngologica, 2004. 124(2): p. 137-143.] and approach steady state as either positive or the negative decaying transient. The time constant of the artifact is on the order of tens of microseconds and the amplitude at 200 μs after pulse presentation can be as large as several hundred microvolts. The artifact scales with amplitude of the stimulation pulse. It is consistent for a given recording and so cannot be removed by multiple measurements. This artifact appears unpredictable with regard to electrode impedances, or which electrodes are used for the recording. The nature of the artifact has not been satisfactorily explained.
A number of methods have been developed to remove the artifact from the eCAP recordings. Some of these methods attempt to eliminate the artifact by characterizing the measurement response with the artifact and subtracting the measurement of the artifact alone. These methods are the alternating polarity (AP) recording protocol [Brown, C. J. and P. J. Abbas, “Electrically Evoked Whole-Nerve Action-Potentials—Parametric Data from the Cat”. Journal of the Acoustical Society of America, 1990. 88(5): p. 2205-2210.], masker-probe (MP) protocol [Abbas, P. J., et al., “Summary of results using the nucleus CI24M implant to record the electrically evoked compound action potential”. Ear and Hearing, 1999. 20(1): p. 45-59], and the template protocol [Miller, C. A., et al., “Electrically evoked compound action potentials of guinea pig and cat: responses to monopolar, monophasic stimulation”. Hearing Research, 1998. 119(1-2): p. 142-154.]. Other methods attempt to predict the double exponential model of the artifact and subtract it from the recording [Klop et al., p. 137-143.]
It is thus seen that there is a need for a system and method that eliminates artifact in eCAP recordings in order to speed up the acquisition and increase the accuracy of eCAP measurements, particularly to (1) detect the artifact (2) eliminate the artifact, and (3) maintain the accuracy of neural response recordings in neural stimulators, e.g., cochlear implants.